The present invention relates to an improved electrophotographic copying apparatus in which electrostatic images on a drum are repeatedly developed to produce a large number of copies of a single original document in a short period of time.
In the most basic electrophotographic copying apparatus a photoconductive drum or belt is electrostatically charged and radiated with a light image of an original document to form an electrostatic image of the document on the drum. A toner substance is then applied to the drum to develop the electrostatic image into a toner image which is subsequently transferred to a copy sheet. The toner image is thermally or otherwise fixed to the copy sheet to provide a permanent copy and any residual toner substance is removed from the drum in preparation for another copying cycle.
Whereas this copying method is satisfactory for copying large size documents, it is wasteful in copying small documents since the majority of the surface of the drum is not used. To increase the copying speed where a large number of copies of a single small document are to be produced, it has been proposed to form a number of electrostatic images of the document spaced form each other about the circumference of the drum and repeatedly develop the electrostatic images and transfer them to respective copy sheets. In this manner, a number of copies equal to the number of electrostatic images on the drum can produced for each revolution of the drum.
Although this method of repeatedly developing an electrostatic image finds its most efficient application with respect to producing a large number of copies of a single small document, it is also beneficial to incorporate the method into a copying apparatus in which the actual copying operation is performed during a first revolution of the drum and the drum is cleaned during a second revolution thereof. In such an apparatus, the same magnetic brush which is used in the developing step during the first revolution of the drum is used to clean the drum during the second revolution thereof. By eliminating the necessity of cleaning the drum after each copy is produced, one copy can be produced for each revolution of the drum thereby increasing the copying speed by 100% in cases where more than one copy is desired from a single document.
In either type of copying apparatus embodying repeated development of an electrostatic image, a problem is encountered in that the electrostatic image can only be developed a certain number of times if acceptable copies are to be produced. This is because the electrostatic image partially discharges during each developing and transfer step. When the charge intensity or voltage of the electrostatic image drops below a certain value, copies of acceptable density and contrast can not be produced.
In prior art copying apparatus of this type, if a large number of copies are to be made of a single document, the apparatus operator must visually determined when the copy quality becomes unacceptable and manually control the apparatus to re-expose the drum for making the remaining copies. This procedure is wasteful since the operator will usually wait until the copy quality becomes unacceptable and throw away the unacceptable copies before initiating another exposure operation.
Such prior art apparatus generally gives no indication when the exposure operation is completed, and the operator must wait until the entire copying operation is completed before removing the original document. This problem is particularly serious in a copying apparatus in which the original document is circulated for repeated exposure. Each time the document is fed through the apparatus the possibility for a jam is created.
Yet another problem exists in copying apparatus utilizing the method of repeated development of a number of electrostatic images of a small original document formed on a drum or belt. Where the number of images on the drum exceeds the number of copies remaining to be made, the developing means will not be de-energized until the drum completes the final revolution. In this manner, at least one electrostatic image will be developed but the resulting toner image will not be transferred to a copy sheet. Instead, the toner image will be transferred onto the surface of a transfer roller or belt and will result in double printing in subsequent copying operations. In addition, the cleaning unit will be overloaded. An even more serious result will occur if the original document is removed prior to a required exposure operation. The drum will not be exposed and a completely black image will be transferred to a copy sheet and/or transfer drum.